A driving motor of which the speed can be continuously varied by means of a control member produces a torque that is nearly independent of the speed. Vehicles and working machines however require variable driving forces to overcome resistance to travel such as inclines, to produce thrust and/or tractive forces or for acceleration. Hence it should be possible to convert the installed driving power as completely as possible into working power in the drive system. This requirement is met by a converter connected between the driving motor and the drive shaft of the drive system, having a speed controller that controls the converter in accordance with the load condition of the driving motor in response to control or regulating signals that correspond to the load condition, so that the speed depression of the driving motor does not exceed a desired amount. Speed(rotation) depression is the name given to the difference between, or ratio of, the speed of the driving motor preset by the control member and the speed which results under the load. A particularly suitable converter is a hydrostatic transmission, because of its constructional advantages and of the possibility of intervention in the adjustment of the hydraulic pump and/or hydraulic motor. A mechanical converter, for example a continuously variable mechanical transmission, can however be used.
A known drive system of the above-mentioned kind is used as a vehichle driving system, particularly for constructional vehicles such as wheel loaders, excavators and bulldozers, for both forward and backward travel, and has a hydrualic control device with an adjusting piston that is centered by means of springs. The adjusting piston is connected to the swivelling rocker of the hydraulic pump. A direction-of-travel valve connected ahead of it determines the direction of adjustment of the adjusting piston and thus the direction of delivery of the hydraulic pump. The extent of the adjusting displacement, and thus the amount of the delivery volume of the hydraulic pump, depends on the magnitude of the control pressure that is produced by an auxiliary group of constant delivery volume, driven synchronously with the hydraulic pump, and acts on the adjusting piston. The delivery flow of the auxiliary pump, which is proportional to the speed, produces a pressure drop at an orifice, which is used to set the control pressure by means of a control piston. Such a control valve, which in the present field is known as a "pressure balance" or "pressure compensator" is described and illustrated in No. DE-PS-22 47 437.
A drive system has also been proposed in which there is a predetermined fixed relationship (control characteristic) between the control member of the driving motor and the adjusting member of the associated converter in order to set the speed and/or delivery thereof,overloading (stalling) of the driving motor being prevented by a load limiting control.
In the drive system described above, if the depression of the driving motor is too great the associated converter is controlled so that the speed of the driving motor does not fall below a specified value, so that, in the case of a internal combustion engine such as a diesel engine, stalling is avoided, and when there is a subsequent change-speed gear a speed range that is appropriate in respect of power ulitization and fuel consumption is aimed at.
All these known kinds of drive systems have a fixed control characteristic whereby the speeds of the driving motor from which the load limiting control comes into operation are fixed.
Thus if for example at a particular position of the gas pedal the permissible speed (rotation) depression of the driving motor (which is defined by the fixed control characteristic) is not exceeded, in the known arrangements the converter is controlled so that as the speed of the driving motor increases the transmission ratio increases, which in the case of a hydrostatic transmission is brought about by an increase in the pump delivery flow.
If, as a result of resistance to travel, the permissible speed depression of the driving motor is exceeded, the converter is no longer controlled proportionally to the speed, as described above, but is then adjusted or regulated so that the speed of the driving motor does not fall below that corresponding to the fixed control characteristic (load limiting control, i.e. anti-stalling control).
The known arrangements have the following disadvantages:
changes in the control and regulating characteristic because of wear of hydraulic control parts, e.g. orifices, spools and the like, or of mechanical control parts such as control members, gas (accelerator) rods and other adjusting rods;
after intervention in the system, for example change of the driving motor, injection pumps, gas rods or hydraulic units, partial or complete resetting is needed;
high setting-up costs;
temperature dependence;
viscosity dependence (in the case of hydraulic control devices);
in the case of a converter with speed-dependent adjustment (e.g. setting of the delivery volume of the pump) the power of the driving motor is used inefficiently, particularly in the lower speed region; and
the maximum power supplied by the driving motor cannot be set.